Unskilled player is a report, but it doesn't make it to the Tribunal. When someone gets reported for being an unskilled player very often, then automated systems in League of Legends take it in consideration for matchmaking purposes.

"Unskilled Reports aren't used in the Tribunal, but are aggregated for matchmaking analyses and adjustments.

We never want to ban a player for Unskilled Play because that makes no sense--everyone has bad games once in awhile, and League of Legends is for every player regardless of them being 500 Elo or 2000 Elo. If a player gets a lot of Unskilled Player reports (above the norm), then we need to figure out what went wrong in the matchmaking that the player got put into the wrong bracket and improve that."